Imagers, including complimentary metal oxide semiconductor (CMOS) imagers and charge-coupled devices (CCD), may be used in digital imaging applications to capture scenes. An imager may include an array of pixels. Each pixel in the array may include at least a photosensitive element for outputting a signal having a magnitude proportional to the intensity of incident light contacting the photosensitive element. When exposed to incident light to capture a scene, each pixel in the array outputs a signal having a magnitude corresponding to an intensity of light at one point in the scene. The signals output from each photosensitive element may be processed to form an image representing the captured scene.
Ideally, all pixels in an imager will produce a signal having the same magnitude when exposed to incident light having the same intensity. In reality, however, under uniform illumination, some pixels in each manufactured imager may produce signals having larger or smaller magnitudes than the average pixel in the imager. Pixels producing signals having larger magnitudes than the average pixel are typically referred to as hot or white pixels and signals having smaller magnitudes than the average pixel are typically referred to as cold or dark pixels. Because the intensity of light at a point in the formed image corresponds to the magnitude of the signal output by the corresponding pixel, hot pixels may appear as white spots in the formed image and cold pixels may appear as dark spots in the formed image. Accordingly, while almost all real imagers have at least a few hot and cold (collectively referred to as “aberrant”) pixels, the effects of these aberrant pixels are not ideal.
Aberrant pixels may exist in real imagers for several reasons. One reason may be physical defects in the pixel. Examples of physical defects may include point defects in the material used to fabricate the pixel array, short or open circuits in the photosensitive element, defects in the readout circuitry and design defects in the reset circuitry. An aberrant pixel may also occur due to non-uniform additional charge—not generated from incident light—exists in the pixel. One such charge may be leakage current. Leakage current refers to electric charges that leak into the sensor wells. Another such charge may be dark current. Dark current refers to a certain level of background electron collection that exists in all pixels due to thermal generation of electron hole pairs. Because the readout circuitry may not distinguish between sources of charge in the photosensitive element, non-uniform leakage current and dark current may be added to or subtracted from the magnitude of the signal output from the pixel, thus making the pixel appear brighter or darker in the produced image than that point actually appeared in the scene.
Because it would be prohibitively expensive and time consuming to require that all imagers produced on a production line have no aberrant pixels, it may be acceptable to produce imagers that contain some aberrant pixels. To counteract the effect of the aberrant pixels in these imagers, the aberrant pixels may be identified and corrected for during image processing.
Common methods of detecting aberrant pixels include comparing the output signal of each pixel with the output signals of neighboring pixels. Different algorithms may use a different number of neighboring pixels or different neighboring pixel locations for this purpose. If the difference falls outside a threshold range, the pixel may be identified as an aberrant pixel and the pixel may be corrected for during image processing. If the difference falls within the threshold range, the value may be output without correction.
One concern in designing aberrant pixel correction algorithms may be to provide a balance between accurately identifying as many truly aberrant pixels as possible and falsely identifying too many truly good pixels as aberrant pixels. The common aberrant pixel algorithms described above may identify too many good pixels as aberrant pixels for at least two reasons. First, images with sharp contrasts may cause common correction algorithms to falsely identify many good pixels in high-contrast regions of the image as aberrant pixels. This is because neighboring pixels in high contrast images may drastically differ in intensity, causing the difference between levels of neighboring pixels to be large. Second, with respect to color image sensors, color filters are typically arrayed over the pixels such that the immediate neighbors of any given pixel are sensitive to different wavelengths of incident light. This may cause the common pixel correction algorithm to identify too many good pixels as aberrant pixels or, at the very least, complicate pixel correction algorithms that may account for the different wavelengths.
For at least these reasons, common pixel correction methods and apparatuses may fail to identify aberrant pixels which should be corrected or they may average out real signal variations through overcorrection, resulting in loss of image sharpness.